Investigation of Helicopter Control Loads Induced by Stall Flutter
UNITED TECHNOLOGIES CORP STRATFORD CT SIKORSKY AIRCRAFT DIV
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An analytical study was conducted to determine if available unsteady normal force and moment aerodynamic test data could be used in conjunction with existing helicopter rotor aeroelastic and variable inflow analyses to provide a method for predicting the stall flutter response of a helicopter rotor blade. For this purpose, incompressible unsteady aerodynamic data for an NACA 0012 airfoil executing pure sinusoidal pitching motions were employed. To apply such data under rotor blade operating conditions where multiharmonic motions and velocity variations exist, the data were expressed as functions of instantaneous section angle of attack, angular velocity, and angular acceleration. In addition, scaling procedures were developed in an attempt to account for the effects of compressibility. Limited application of the resulting analysis to define the aeroelastic characteristics of several blade designs showed that significant self-excited torsional oscillations of the stall flutter type could, in fact, be predicted for certain combinations of flight conditions and blade designs. Correlation studies, to evaluate the ability of the analysis to predict control loads, were performed with CH-53A maneuvering flight test data and with level flight test data from the NH-3A S-61F.